Development of miniature cruciform specimen and testing machine for multiaxial creep investigation

•Biaxial tension-type testing machine is developed for multiaxial creep investigation.•Miniature cruciform specimen is designed for multiaxial creep testing.•Non-contact displacement measuring method for the miniature specimen is developed.•Specimen surface conditions during tests can be observed with developed apparatus. High temperature components such as boiler tubes and jet engine turbine blades undergo multiaxial creep damage due to extreme temperature conditions and their complex shapes. A multiaxial creep investigation is required for the safety of high temperature components in the design phase of the manufacture of such a component. However, there are only a few commercial testing machines that can conduct multiaxial loading at high temperatures. A miniature cruciform specimen having a plane stress condition gauge section that is 5 mm square was designed using finite element (FE) analysis. Further, a biaxial testing machine capable of tensile loading was designed to conduct multiaxial creep tests. The testing machine had a 2 kN loading capacity and a 1 kW furnace. We also developed a non-contact displacement measuring method for the miniature specimen, which had an excessively small gauge section to permit the attachment of a mechanical extensometer. The proposed method uses a conventional optical camera to capture some images of the surface. We obtained the displacement value of the specimen, by tracking the trace of the target marks painted on the surface of the specimen. The measured strain value that was obtained from the non-contact displacement measuring method corresponds to the strain obtained by the mechanical method at room and high temperatures. By using the developed multiaxial creep testing machine and the non-contact observation system, we can investigate not only the deformation of the testing specimen, but also the surface conditions of materials during the creep test.